Less than $1 million of that was spent by Romney's official campaign, while the pro-Romney superPAC Restore Our Future — which has almost exclusively engaged in negative advertising this year — spent $5.7 million.

That's compared to $220,000 spent on ads last week by the superPAC supporting former Pennsylvania Sen. Rick Santorum.

In anticipation of Tuesday's big showdown in Ohio, Romney and his supporters were spending to erase Santorum's lead in the state.

"Given what Romney spent in Ohio in the last week, at least we have one hypothesis about why he closed the gap," says John Geer, a political scientist at Vanderbilt University who specializes in political advertising. "He simply outspent Santorum by, you know, a dramatic amount."

Still, Romney only won Ohio by less than 1 percent.

SuperPACS Doing The Dirty Work

NPR's analysis uses superPAC spending reports from the Federal Election Commission as well as data from the media tracking firm Kantar Media, as reported by The Washington Post. But what the analysis can't measure is the effectiveness of superPAC advertising.

Political scientist John Geer says the Romney superPAC may be strategically boxed in. He says the logical attack against Santorum would be that he's extremely conservative, but in this year's Republican contests extremely conservative is what a lot of voters want.

"So instead they have to talk about earmarks and some other things that just don't really jibe," Geer says. "I think the spending that's going on, especially the negative ads, probably aren't having very much effect because they're picking at small things."

Then there's the argument that superPAC attack ads are having an impact on the contest — just not a very good one.

Benjamin Bates, a professor of communications at Ohio University, says candidates always want a surrogate speaker to make attacks on their behalf. This year, the newly created presidential superPACs have embraced that mission.

"By letting the superPAC take on that role, the surrogate is able to do all the dirty work on behalf of the candidate, which helps protect the attacker's image," Bates says.

Advertising by all of the superPACs has been overwhelmingly negative. But there's a problem there: The relentless negativity seems to discourage some people from voting.

According to Bates, turnout in Ohio was expected to go up this year compared to 2008 because of the competitive nature of the GOP race. Instead, turnout dropped about 5 percent. Bates thinks he knows why.

"The superPACs are definitely doing their job," he says. "Their job is to introduce negativity and to get people to stay home, and they seem to have done that very, very well."

As the primary battles rage on, that job continues. The pro-Romney superPAC has told the FEC that so far it has spent $1.3 million in Alabama and Mississippi — which hold their primaries next Tuesday — and the superPAC that backs former House Speaker Newt Gingrich reported spending $1.4 million in those states and Kansas, which votes on Saturday.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

MELISSA BLOCK, HOST:

Mitt Romney's six primary wins on Super Tuesday did not come cheap. An NPR analysis of spending shows that just last week the Romney campaign and the Pro-Romney superPAC pumped millions of dollars into TV ads.

NPR's Peter Overby has our story.

PETER OVERBY, BYLINE: Romney's own campaign didn't spend all that much on TV last week, less than a million dollars, but the Pro-Romney superPAC, which has done almost nothing but negative advertising all year long, spent $5.7 million on TV for the week. The big showdown was in Ohio between Romney and former Pennsylvania Senator Rick Santorum.

Last week, although the Santorum campaign wasn't on Ohio TV at all, the Pro-Santorum superPAC spent $220,000 on commercials, but that was only a bit more than one-tenth of what was reported by Romney campaign and superPAC, as Romney fought to erase Santorum's lead in Ohio.

John Geer is a political scientist at Vanderbilt University. His specialty is political advertising.

JOHN GEER: Given what Romney spent in Ohio in the last week, at least we have one hypothesis about why he closed the gap. He simply outspent Santorum by, you know, a dramatic amount.

OVERBY: And Romney won Ohio by less than a percentage point. The NPR analysis uses superPAC spending reports to the Federal Election Commission, plus data from the media tracking firm Kantar Media CMAG, as reported by the Washington Post.

But there's one thing the analysis can't measure, that's the effectiveness of the advertising. At Vanderbilt, John Geer says the Romney superPAC may be boxed in strategically. He says the logical attack against Santorum would be that he's extremely conservative, except that, in this year's Republican primaries, extremely conservative is what a lot of voters want.

GEER: So, instead, they have to talk about earmarks and some other things that just don't really jive and so I think the spending that's going on, especially the negative ads, probably aren't having very much effect because they're picking at small things.

OVERBY: But there's also an argument that all of the superPACs with their attack ads have had an impact on the contest and not a good one. Benjamin Bates is a professor of communication studies at Ohio University. He says candidates always want a surrogate speaker to make attacks on their behalf. This year, the newly created presidential superPACs have embraced that mission.

BENJAMIN BATES: By letting the superPAC take on that role, the surrogate is able to do all the dirty work on behalf of the candidate, which helps protect the attacker's image.

OVERBY: Advertising by all of the superPACs has been overwhelmingly negative, but here's the problem: The relentless negativity seems to discourage some people from voting. Bates says that turnout in Ohio was expected to go up this year compared to 2008. That's because the GOP race is so competitive. Instead, turnout dropped about five percent. Bates thinks he knows why.

BATES: The superPACs are definitely doing their job. Their job is to introduce negativity and to get people to stay home, and they seem to have done that very, very well.

OVERBY: And that job is continuing as the primary battles grind on. The Pro-Romney superPAC has told the FEC that, so far, it has spent $1.3 million in Alabama and Mississippi. And the superPAC that backs former House Speaker Newt Gingrich reported spending $1.4 million in those states and Kansas.